第三章单烯烃(6学时)修改版.ppt
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1、第三章 单烯烃Chapter 3Alkenes(6 hours),Classification of Olefines 烯烃的种类,Contents,3.1 Structure of Alkenes3.2 Isomerism and nomenclature3.3 Physical properties of alkenes3.4 Chemical properties of alkenes3.5 inductive effect 3.6 Mechanism of electrophilic addition 3.7 ethene and propene3.8 Preparation of a
2、lkene3.9 Fossil oil,Calculating the Degree of Unsaturation(不饱和度),中文名称:“烯烃的烯意思是少”,Because of its doube bond,an alkene(CnH2n)has fewer hydrogens than an alkane(CnH2n+2)with the same number of carbons,and is therefore referred to as unsaturated.,=1,=1,=2,In general,each ring or double bond in a molecul
3、e corresponds to a loss of two hydrogens from the alkane formula(CnH2n+2)The degree of unsaturation-the number of rings and/or mutiplebonds present in the molecule.,1 ring,1 ring1 C=C,1 C=C,3.1 Structure of Alkenes,3.1.1 乙烯的结构(Ethylene,Ethene),现代物理方法证明,乙烯分子的所有原子在同一平面上,(Planar or flat)其结构如下:,碳原子的SP2杂
4、化,SP2杂化轨道示意图,SP2杂化的特点,SP2 轨道与P轨道的关系,乙烯分子的形成,双键(C=C)=键+键键能 CC 346kJ/mol C=C 610kJ/mol 键键能=610-346=264kJ/mol,Bonding in Ethylene,molecular orbitals,键的特点,不如键牢固(Sideways overlap)。不能自由旋转。电子云沿键轴上下分布,不集中,易极化,发生反应。不能独立存在。,The bond must break for rotation to take place around a carbon-carbon double bond,bond
5、(p orbitals are parallel),Broken bond after rotation(p orbitals are perpendicular),3.1.2 烯烃的结构特征,1 双键碳是sp2杂化。2 键是由p轨道侧面重叠形成。3 由于室温下双键不能自由旋转,所以烯烃有几何(Z,E)异构体。,cis-and trans-isomers are different compounds with different physical properties!,3-D Molecules:,Cis and trans isomers can be interconverted,3.
6、1.3 Cis-Trans Isomerism in Alkenes,cis-2-Butene,trans-2-Butene,The cis isomer has the two methyl groups on the same side of the double bond,and the trans isomer has the methyl group on opposite sides.,The requirement for cis-trans isomerism in alkenes,These two compounds are identical;they are not c
7、is-trans isomers.,These two compounds are not identical;they are cis-trans isomers.,Conclusion:Compounds that have one of their carbons bonded to identical groups cant exist as cis-trans isomers.Only when both carbons are bonded to two different groups are cis-trans isomers possible.,3.2.1 烯烃的同分异构现象
8、,烯烃的同分异构现象比烷烃的要复杂,除碳干异构外,还有由于双键的位置不同引起的位置异构和双键两侧的基团在空间的位置不同引起的顺反异构。,3.2 烯烃的异构和命名,1)构造异构(Constitution isomerism),碳干异构,1-Butene2-Butene2-Methylpropene,位置异构,2)顺反异构(Cis-Trans Isomerism),产生顺反异构体的条件:构成双键的任何一个碳原子上所连的两个基团要不相同。必要条件:双键充分条件:连接基团不同,顺反异构体的物理性质不同,因而分离它们并不很难。,1.Common name,3.2.2 烯烃的命名(Naming Alken
9、es),Ethene,2-Methylpropene,2-Methyl-1,3-butadiene,These common names often used and are recognized by IUPAC.,2.Systematic Nomenclature of Alkenes,1)选择含碳碳双键的最长碳链为主链,为某烯。2)从最靠近双键的一端开始,将主链碳原子依次编号。3)将双键的位置标明在烯烃名称的前面(只写出双键碳原子中位次较小的一个)。4)其它同烷烃的命名原则。,2,5-二甲基-2-己烯(2,5-dimethyl-2-hexene),顺反异构体的命名,1)顺反命名法:即在系
10、统名称前加一“顺”或“反”字。,The cis-trans naming system only works with disubstituted alkenes-compounds that have two substituents other than hydrogen on the double bond.,Trans-,Cis-,Cis-trans naming system,With trisubstituted and tetrasubstituted double bonds,the cis-trans naming system does not work,a more ge
11、neral method is needed for describing double-bond geometry.,Trisubstituted or tetrasubstituted alkenes-compounds that have three or four substituents other than hydrogen on the double bond.,2)E,Z Designation and Sequence Rules,According to the E,Z system of nomenclature,a set of sequence rules is us
12、ed to assign priorities to the substituent groups on the double bond carbons.When each double bonded carbon atom is considered separately,the sequence rules are used to decide which of the groups attached to each is higher in priority.If the higher-priority groups on each carbon are on the same side
13、 of the double bond,the alkene is designated Z.If the higher-priority groups are on opposite sides,the alkene is designated E.,Z,for German zusammen,meaning“together”E,for German entgegen,meaning“opposite”,分别按“次序规则”比较两个双键碳原子上的取代基团,定出较优基团,如果两个双键碳上的较优基团位于双键的同侧,则为Z构型,反之为E构型。,Z是德文 Zusammen 的字头,同一侧的意思。E是
14、德文 Entgegen 的字头,是相反的意思。,2)Z、E命名法,(E)-2-Bromo-1-chloropropene,2)E,Z Designation and Sequence Rules,E double bond(Higher priority groups are on opposite sides),Z double bond(Higher priority groups are on same side),Sequence Rules:,Considering the double-bond carbons separately,look at the two atoms di
15、rectly attached to each and rank them according to atomic number.,(Z)-2-Chloro-2-butene,(E)-2-Chloro-2-butene,(Cis)-,(Trans)-,2.If a decision cant be reached by ranking the first atoms in the substituent,look at the second,third,or fourth atoms away from the double-bond carbons until the first diffe
16、rent is found.,Sequence Rules:,Lower Higher,Lower Higher,3.Multiple-bonded atoms are equivalent to the same number of single-bonded atoms.,(Z)-3-甲基-4-异丙基-3-庚烯,(E)-3-Methyl-1,3-pentadiene,E configuration,Z configuration,CH3CH=CH1-丙烯基1-propenyl,CH2=CHCH22-丙烯基或烯丙基allyl,CH2=C1甲基乙烯基或异丙烯基isopropenyl,CH2=C
17、H乙烯基Vinyl,烯基 烯烃分子去掉一个氢原子后剩下的一价基团。,3)几个重要的烯基(alkenyl),二 亚基,H2C=CH3CH=(CH3)2C=亚甲基 亚乙基 亚异丙基Methylidene ethylidene isopropylidene CH2 CH2CH2 CH2CH2CH2 亚甲基 1,2-亚乙基 1,3-亚丙基Methylene ethylene(dimethylene)trimethylene,两种亚基:中文名称通过前面的编号来区别,英文名称通过词尾来区别,有两个自由价的基称为亚基。,随分子量的增加而升高;,碳原子数相同,直链烯烃的比带支链的高;,物质状态,烯烃在室温常压
18、下:1C4C(g),5C16C(l),17C(s),Boiling point 沸点 bp.,-6.3,-6.9,bp./,3.3 The Physical Properties of Alkenes烯烃的物理性质,碳链相同,双键向中间移动沸点升高;,双键位置相同,顺式的烯烃沸点高。,-6.3,3.7,bp./,0.88,原因:顺-2-丁烯偶极矩为0.33D,反-2-丁烯偶极矩为0。,-138.9,-105.6,mp./,melting point 熔点 mp.,也随分子量的增加而升高;,碳链相同,双键向中间移动熔点也升高;,同分异构体中,对称性大的烯烃熔点高。,Solubility 溶解度,
19、烯烃的相对密度都小于1,但比相应的烷烃大。,Density 相对密度,在水里的溶解度很小,但也比烷烃大;,在亚铜盐或银盐的水溶液中,溶解度很大。,非常活泼C=C,C=C 加成 饱和烃,氧化-发生在富裕电子部位,-H:烯丙位氢活波,可被卤代.,3.4 烯烃的化学性质 p48,3.3 烯烃的物理性质(略)p47,3.4 The Chemical Properties of Alkenes烯烃的化学性质,-C 是与官能团相连的碳。,-H,-C上的氢,-H 较其他氢活泼。,非常活泼C=C,Structure of Alkene,一、烯烃的亲电加成反应,Electrophilic addition,在烯
20、烃分子中,由于电子具流动性,易被极化,因而烯烃具有供电子性能,易受到缺电子试剂(亲电试剂)的进攻而发生反应,这种由亲电试剂的作用而引起的加成反应称为亲电加成反应。,Alkenes behave as nucleophiles(Lewis bases)in polar reactions.The carbon-carbon double bond is electron-rich and can donate a pair of electrons to an electrophile(Lewis acids).,试剂reagent,亲电试剂(亲电体)Electrophilic reagent(
21、electrophiles),亲核试剂(亲核体)Nucleophilic reagent(nucleophiles),These are electrophiles because they can accept a pair of electrons,These are nucleophiles because they have a pair of electrons to share!,亲电试剂,亲核试剂,1.Electrophilic Addition of HX to Alkenes,1)The order of reactivity of hydrogen halides to a
22、lkenes:HI HBr HCl HF(HF的加成无实用价值):,The reaction occurs in two steps and involves electrophile-nucleophile interaction.,2)Mechanism:,The electrophilic addition reaction of ethylene and HBr.,决速步骤,Reaction progress,Energy,Ea1,Transition state,TS-1,Ea2,TS-2,Carbocation intermediate,Reaction energy diagra
23、m for the two step electrophilic Addition of HBr to alkene.The first step is slower than The second step.,3)Markovnikovs Rule:,Two possible products,Not formed,Sole product,unsymmetrical alkenes,Regioselective reaction is a reaction in which two isomeric products could be obtained,but one predominat
24、es.,Markovnikovs rule:,In the addition of HX to unsymmetrical alkenes,the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more substituents.,The reason for Markovnikovs rule:,More stable,less stable,Progress of carbocation formation,Energy diagram for the
25、 protonation of 2-methypropene,E,E(1o),E(3o),Transition state,Reaction takesthis course,Summary:,Electrophilic addition to an unsymmetrically substituted alkene gives the more highly substituted carbocation.2.A more highly substituted carbocation is more stable than a less highly substituted one.,Ca
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